Improve ocl cvt_color performance for the following conversions: RGB<->B...

...GR, RGB->Gray, RGB<->XYZ, RGB<->YCrCb, RGB<->YUV, and mRGBA<->RGBA.

The improvement was done basically by processing more than 1 pixel by each work-item and using vector's operations.
new performance tests were added

check_regression=*Color*